During the past year we have continued to delineate the region of the guinea pig myelin basic protein that is highly encephalitogenic in Lewis rats. We have found that a 11-residue region, Gln-Lys-Ser-Gln-Arg-Ser-Gln-Asp-Glu-Asn-Pro, is highly active at a level of 0.5 nmole (0.66 microgram). Staphylococcal proteinase has been used to cleave specifically the basic protein at aspartyl and glutamyl residues. The resulting peptides are being used to define the immunologically active regions of the protein. A study on the hydrodynamic properties of the basic protein has so far shown that the protein undergoes considerable contraction and reduction in asymmetry at high pH as well as in concentrated NaCl solutions. BIBLIOGRAPHIC REFERENCES: Martenson, R.E., Deibler, G.E. and Kramer, A.J.: Reaction of peptide 89-169 of bovine myelin basic protein with 2-(2-nitrophenylsulfenyl)-3 -methyl-3'-bromoindolenine. Biochemistry 16: 216-221, 1977. Martenson, R.E., Nomura, K., Levine, S. and Sowinski, R.: Experimental allergic encephalomyelitis in the Lewis rat: further delineation of active sites in guinea pig and bovine myelin basic proteins. J. Immunol. 118: 1280-1285, 1977.